One important method for recovering carbon dioxide is the absorption of the carbon dioxide from lean carbon dioxide sources, such as flue gases from combustion sources, from the gas stream into an amine recovery solvent and the subsequent separation and recovery of the carbon dioxide from the recovery solvent.
Two different amine based carbon dioxide recovery systems are known and practiced commercially for the absorption of carbon dioxide from lean sources at or slightly above atmospheric pressure. In one such system a relatively high concentration of amine is used in the recovery solvent. However, because a high concentration of amine in the recovery solvent causes corrosion problems in the downstream processing, the recovery solvent must contain effective levels of inhibitors. This increases both the cost and the complexity of the carbon dioxide recovery.
In another amine based carbon dioxide recovery system, a relatively low amine concentration is used in the recovery solvent. This system avoids the costly and complicated need to use inhibitors in the recovery solvent but at the expense of larger equipment and higher energy requirements. There is a clear need for an amine based carbon dioxide recovery system which enables a high rate of carbon dioxide recovery with lower capital and operating costs while avoiding the need to employ inhibitors in the recovery solvent.
Amine blends have been used before for the absorption of carbon dioxide from gases containing several other gas species. However, in these instances, the feed gas is typically at a pressure in excess of 100 pounds per square inch absolute (psia) with a carbon dioxide partial pressure greater than 25 psia. Examples of such gases include natural gas, coke-oven gas, refinery gas and synthesis gas. Typically such amine blends employ a tertiary alkanolamine such as methyldiethanolamine in a high concentration as the main component and also employ a small amount of a rate promoting agent such as monoethanolamine, diethanolamine or piperazine. The present invention is directed to carbon dioxide recovery from feed gases at a pressure of about 14.7 to 30 psia and having a carbon dioxide partial pressure typically within the range of from 0.3 to 10 psia. The amine blends useful for absorption at high pressure do not work well for low pressure systems.
Accordingly it is an object of this invention to provide an improved amine based carbon dioxide recovery system which enables high recovery of carbon dioxide from a low pressure system.